The pushbuttons on the market to the best of our knowledge, are constituted by a switch and a display. The switch generally is mechanical and has a limited life because of the use of mechanical contacts and springs. Mechanical and electrical switching jumps are unwanted negative effects. Such effects generate false signals to be eliminated by electronic means.
Until now in such pushbutton, the part activated by the operator is illuminated by lamps which generate a base light similar to white light and to obtain different colors, filters and optical corrections are employed. These lamps do not permit changes in the emitted light color.
Operating rules and the human engineering prescriptions require the use of lighting of different colors and brightness. For instance, white light is used to permit the reading of information and the identification of the command by the operator; red light is used to read out alarm states and variable lighting (green, blue, amber, etc) can be used depending on the operative conditions.
Lamps have a limited useful life and require access into the illuminated pushbutton for the replacement of failed lamps.
To verify the presence of failed lamps it is necessary to implement a lamp test function that the operator activates using a specific command. The off state of the lamps, creates ambiguity between the condition of failed lamps or a deactivated state of a lamp.